CEL Code 420 and Replacement Catalytic Converter / Exhaust System help

[YotaFun]

I don't know about the rear cat doing most of the work.
Usually when we get a 4Runner in with the P0420 code, we replace the front cat.

Brian, you did the universal on the 2000?

[MTL_4runner]

the front cat is steel and looks like its wound in a circle. the rear cat is the one doing most of the work, with the expensive metals, and honeycomb shape. i replaced both of mine recently with a single universal cat and yes the sound is fun and no check engine light since

I'd be very surprised if this was the case since the second O2 sensor should be there to verify the front converter is working properly. The second cat is really there to clean up anything the first converter missed....thus it results in lower emissions (which is why California vehicles had this setup first).

[20005spd]

my truck had factory. an air/fuel sensor, then 2 cats in a row, then an 02 sensor. at least around NE you can put whatever cat makes that rear 02 sensor happy and all is good. we dont do tailpipe tests on anything 96+. OBD2 will find an emissions problem and cause the readiness monitors to read incomplete which is sent to the inspection machine via OBD2 connector dictating whether or not you get a sticker.

[YotaFun]

Jamie's right on this one Brian, at least out of all the Runners that have been coming in oo+ i have seen that way, can you get a pic if you still have it?

[MTL_4runner]

Brian, you are right on the configuration, but not on the operation. The converter closest to the engine gets the hottest and by default does the most work to "clean" the tailpipe emissions. The second O2 sensor is at the end of both converters but again it really is checking the front converter since that one is far more likely to wear out than the one further downstream.